Increasing communications volume has increased the demand for higher-power solid-state devices in both terrestrial base stations and satellite-communications (satcom) systems. The required output power for GaAs field-effect transistors (FETs) has been pushed higher for both base-station and earth-station applications. Some operators simply want to transmit their signals further to extend their services to a wider area, while others want to send more data over the existing frequency spectrum by using a high-level modulation scheme combined with a complicated multiple-access scheme, which requires high linearity or high peak power capability. Although GaAs FET technology has been a proven performer in these applications for several decades, alternative device technologies may provide the higher levels of performance needed for these expanding communications systems.
In GaAs technology, Toshiba has developed 90-W devices for C-band applications as well as 30-W devices for Ku-band systems. Still, communications systems designers have sought even higher power levels at these frequencies. Over the past two decades, Toshiba has been utilizing GaAs-based technology with different structures such as metal-epitaxial-semiconductor FET (MESFET) topologies, high-electron-mobility-transistor (HEMT) structures, and heterojunction FETs (HFETs) in quest of achieving higher solid-state power levels at microwave frequencies. Unfortunately, GaAs technology appears to be reaching practical material property boundaries for heat dispersion, limiting the amount of power that can be produced from a given die size.
Since 2003, Toshiba has researched candidates for next-generation materials for high-power microwave amplification. Silicon carbide (SiC) and gallium nitride (GaN) were the strongest early nominees. Considering the core competency of Toshiba for microwave products, good performance at higher frequencies (C-band and above) was a critical requirement. Of the two materials, Toshiba has chosen to focus on GaN, the material perhaps best known for the blue light-emitting diode (LED). In addition to its photonic properties, GaN provides excellent characteristics as a microwave power device, including:
- High electron mobility
- High breakdown voltage
- High temperature operation
The research was revealed that GaN offers higher saturated electron velocity and higher breakdown-voltage performance than GaAs. Because of these characteristics, GaN should be able to provide improved performance over GaAs devices, and operate at frequencies beyond the microwave range. In addition, GaN power amplifiers can operate at higher temperatures than GaAs devices, making use of its wide-bandgap structure.
Toshiba recently announced development of a GaN power FET that far surpasses the operating performance
of the GaAs FETs widely used today in base stations for terrestrial and satellite microwave communication. The new transistor achieves output power of 174 W at 6 GHz, the highest level of performance yet reported at this frequency. This breakthrough performance improvement was realized by optimizing the epitaxial layer and chip structures for 6-GHz-band operation and by adopting a four-chip combination structure to minimize heat buildup. Figure 1 shows a structural model of the high-power GaN HEMT.
GaN Power HEMT
The key points for development of a GaN HEMT are as follows:
- Selecting a base substrate.
- Improving the device process, for example, by reducing the gate leakage current, forming good ohmic electrodes, and controlling potential conditions for current collapse.
- Combining multiple die inside a single package to achieve desired power while considering heat dissipation in mounting the chips.
For a GaN HEMT structure, a crystal-wafer such as sapphire, SiC, or silicon (Si) is employed as a base substrate. The GaN and AlGaN are added as growth layers by molecular-beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) method. Here, the GaN layer and AlGaN layer are formed sequentially (epitaxial growth) for the structure of the substrate. A GaN crystal wafer would be preferable for the base substrate; however, the current technology is unable to produce such a large wafer, so the base substrates listed in the table are currently used as substitute materials.
Each of the substrate options shown in the table has merits and short-comings. In this case of the new GaN device, a SiC substrate wafer was employed because of its heat conductivity and crystal quality on the growth layer for realization of a higher power device.
Improving The Process
GaN exhibits high breakdown voltage. However, gate leakage current can occur between both electrodes or inside of the semiconductor crystal due to the process, impairing high-voltage operation. The path of the leakage current is considered to be on the border of AlGaN layer and dielectric passivation film, or to run into the substrate via some defective crystal in AlGaN layer.
Defective crystals called " rearrangements" or "micropipes" are still found in SiC substrates. The process was visualized using an optical surface analyzer, identifying that the defective crystal on a SiC substrate spreads to a AlGaN layer or GaN layer. The optical surface analysis and visualization made it easier to compare the defective crystal with the fabricated device one by one by superimposing the image from the analyzer on the GaN power HEMT pattern image formed on the growth layer (Fig. 2).
Figure 3 shows the result of comparing the gate leakage current of HEMT on the defective substrate with that of HEMT on the good substrate. This analysis indicates that reducing the defective crystal spread from the crystal wafer to the growth layer is the most important way to improve the device characteristics.
In the GaN HEMT development, the leakage current from the gate electrode or from the surface of AlGaN layer was reduced by optimizing the composition of metals forming the gate electrode and the process before forming the electrodes. The solution for reducing the leakage current through the growth layer would be an improvement of the crystal wafer itself. Instead, a GaN power HEMT formed on a substrate free of defective crystal was selected for the device development.
Similar as for GaAs devices, reducing the contact resistance between the drain and source electrodes is critical key for higher-performance device fabrication with GaN. Since GaN is a low-reactive material, its characteristics vary depending on the types of metals used for electrodes, the process used before forming the electrodes, and the heat treatment. In this development, Ti system electrodes having a laminated structure-were adopted and annealed by rapid thermal anneal (RTA) for short-duration heat treatment at high temperatures. By optimizing the electrode structure and carefully selecting the temperature for anneal, it was possible to form ohmic electrodes for drain and source with contact resistance under 10 - 5 Ω cm2.
Current instability called "current collapse" is often observed in GaN HEMT devices. This phenomenon occurs in three-port devices like HEMT formed on a GaN substrate because drain current decreases as the drain voltage increases with a fixed gate voltage; it is a very significant problem for high-power devices. The cause of current collapse is still a mystery, but one theory gaining popular acceptance is that the electron trapping level is related to the phenomenon, and it is caused from defects on the surface of GaN semiconductors.
In the GaN device development program, efforts were made to suppress such current collapse by means of finding out and adopting experimentally the process and equipment conditions to prevent damage to the surface of the semiconductors before forming the gate electrode and at the formation of the surface passivation film.
As mentioned above, GaN HEMT devices are capable of operating with 10 times the power density of GaAs devices. Yet, the power-added efficiency (PAE) is almost the same between them, and heat increases as the power increases. A GaN HEMT can operate at higher temperatures because its bandgap is broader than GaAs. But at the point of heat resistance with other parts comprising the transistor, the structure and size of a GaN device must be carefully evaluated to optimize the power density and heat radiation.
Figure 4 shows an external view of a fabricated GaN power HEMT. Four GaN chips were mounted in a highpower transistor package. This particular-layout, which prevents the concentration of heat on any one HEMT die, yielded the best performance for the combined GaN device die.
The input/output (I/O) performance characteristics of the newly developed 6-GHz GaN HEMT are noteworthy. With 15 V drain voltage, the device achieved saturated output power of +47.9 dBm (61.7 W) under continuous-wave (CW) operation at 6 GHz and saturated output power of +48.5 dBm (70.8 W) in pulsed opeation (using a 100-µs pulse width and 20-percent duty cycle). The linear gain was 11.5 dB in CW operation and 13.2 dB in pulsed operation.
Differences in saturated power (0.6 dB) are due in part to the difference in operating temperatures. The device achieved saturated output power of +52.4 dBm (174 W) when operating with 25 V drain voltage in pulsed operation (using a 20-µs pulse width and 2.5-percent duty cycle). with linear gain of 13.2 dB and power-added efficiency of 34 percent.
The drain voltage dependence of the device's I/O power characteristics at 6 GHz in pulsed operation (using a 100-µs pulse width and 20-pecent duty cycle) indicates that the output power increases as the drain voltage is increased, demonstrating the great potential of a GaN power device compared to a GaAs MESFET device.
The successful development of a GaN HEMT power device capable of more than 150 W output power at Cband shows the potential of GaN technology and the fact that it is suitable for high-power transistors at microwave frequencies. There still problems to be overcome, such as material defects, current collapse, and effective thermal-design optimization, although progress is being made on the commercialization of GaN power HEMT devices for existing and emerging terrestrial and space-based communications systems.